The invention relates to a method of recovering precious metals from ore material containing such metals.
In this specification the expression "ore material" is intended to include any corresponding naturally occurring ore or ore material, any residues in ore mining and also any industrial residues which contain precious metal. Moreover, the expression "precious metal" encompasses gold, silver and platinum.
For the recovery of precious metals it has long been known to use so-called "heap leaching", which is regarded as an economical method of recovering precious metals from oxidic precious metal ores. In this recovery of precious metals a type of pile or heap consisting of broken and ground ore particles, in which all ore particles are in close surface contact with neighbouring ore particles, is treated with a leaching agent or leaching fluid, such as for example a cyanide solution, which is intended to dissolve the contained precious metal. Before the ore is placed on the heap it is comminuted in order to reduce the particle size and to increase the surfaces of all the ore particles. The comminution is normally carried out in at least one comminution stage or zone or can also be carried out in a roller crushing arrangement, as is described in U.S. Pat. No. 4,960,461. After this, oxidic ore is comminuted in a high-pressure roller press, a binding agent having previously been added to it so that proportions of fine material from the comminution process can be bound to coarser particles in order to make the heap permeable to the leaching fluid. In this known method for this leaching the comminuted product coming out of the high-pressure roller press is delivered directly to the heap without further treatment.
In order to achieve the maximum efficiency for the recovery of precious metal in this heap leaching or in order to ensure the most economical recovery possible, the heap is normally very large and is exposed directly to the atmosphere. The leaching fluid is poured or sprayed directly onto the heap and penetrates the heap, so that it dissolves compounds containing precious metal with which it comes into contact. This charged leaching fluid is collected for further processing.
Such a heap leaching is used only for the treatment of oxidic ores and has not proved effective for the efficient recovery of precious metals from non-oxidic, refractory and semi-refractory precious metal ores.
Since the supply of directly leachable oxidic ores containing precious metal is becoming smaller, the commercial recovery of precious metals from semi-refractory and refractory ores is becoming increasingly important. Precious metal which is contained in non-oxidic ores is normally in the form of sulphides and natural precious metal, and/or it is completely encapsulated in an impermeable gangue matrix. Ores of this type normally require fine comminution (fine grinding) as well as a concentration of material and/or oxidative treatment in order to make them more accessible to leaching.